All it takes is one dirty or loose connection to slow a 6V starter down dramatically. There's not much way for debris to get inside a starter from contamination in the bellhousing. Try this- - - -just push the start switch without engaging the pinion gear to see how the starter free spins. If it winds up freely, there's no real need to dig into it. If you do decide to go further, use a mild solvent like mineral spirits to degrease everything, do not attempt to remove the field coils from the housing, and replace the brushes and bushings while you've got it apart.
Jerry
